OverviewThe weakest solar cycle in more than a century continues to plunge towards the next solar minimum phase. There are currently two lonely sunspot regions visible on the sun (above) and the general trend for the next few years is likely to be for little in the way of solar activity as cycle 24 moves away from its maximum phase and towards the beginning (solar minimum) phase of solar cycle 25. In fact, there will be a growing chance for completely spotless days on the sun during the next few years as the next solar minimum phase is approached. There has been only one completely spotless day on the sun since 2011. The current solar cycle is the 24th since 1755 when extensive recording of solar sunspot activity began and is the weakest solar cycle with the fewest sunspots since cycle 14 peaked in February 1906.

Moving from solar maximum to solar minimumThe sun goes through a natural solar cycle approximately every 11 years. The cycle is marked by the increase and decrease of sunspots which are visible dark regions on the sun’s surface and cooler than surroundings. The greatest number of sunspots in any given solar cycle is designated as the “solar maximum" and the lowest number is referred to as the “solar minimum” phase. We are currently more than seven years into Solar Cycle 24 and it appears the solar maximum of this cycle was reached in April 2014 during a spike in activity (current location indicated by arrow above). Solar cycle 24 is now heading towards the next solar minimum phase which would be the beginning of solar cycle number 25. Contrary to popular belief, space weather does not cease during solar minimum phases and there are actually many interesting changes. For instance, the upper atmosphere of Earth collapses, allowing space junk to accumulate around our planet. The heliosphere shrinks, bringing interstellar space closer to Earth. And galactic cosmic rays penetrate the inner solar system with relative ease during solar minimums.

Going back to 1755, there have been only a few solar cycles in the previous 23 that have had a lower number of sunspots during its maximum phase. The peak of activity in April 2014 was actually a second peak in solar cycle 24 that surpassed the level of an earlier peak which occurred in March 2012. While many solar cycles are double-peaked, this is the first one in which the second peak in sunspot number was larger than the first peak. The sunspot number plot (above) shows a clear weakening trend in solar cycles since solar cycle 22 peaked around 1990. The last solar minimum phase lasted from 2007 to 2009 and it was historically weak. In fact, it produced three of the most spotless days on the sun since the middle 1800’s (bar graph below).

Top "sunspotless" days since 1849; last solar minimum produced 3 of these years

Consequences of weak solar cyclesThere can be important consequences from weak solar cycles; especially, if they are part of a long-term pattern. First, this particular weak solar cycle has resulted in rather benign “space weather” in recent times with generally weaker-than-normal geomagnetic storms. By all Earth-based measures of geomagnetic and geoeffective solar activity, this cycle has been extremely quiet. However, while a weak solar cycle does suggest strong solar storms will occur less often than during stronger and more active cycles, it does not rule them out entirely. In fact, the famous "superstorm" Carrington Event of 1859 occurred during a weak solar cycle (number 10). In addition, there is some evidence that most large events such as strong solar flares and significant geomagnetic storms tend to occur in the declining phase of the solar cycle. In other words, there is still a chance for significant solar activity in the months and years ahead.

400 years of sunspots with Maunder and Dalton Minimums; courtesy wikipedia

Second, it is pretty well understood that solar activity has a direct impact on temperatures at very high altitudes in a part of the Earth’s atmosphere called the thermosphere. This is the biggest layer of the Earth’s atmosphere which lies directly above the mesosphere and below the exosphere. Thermospheric temperatures increase with altitude due to absorption of highly energetic solar radiation and are highly dependent on solar activity.

Finally, if history is any guide, it is safe to say that weak solar activity for a very prolonged period of time (several decades) can have a cooling impact on global temperatures in the troposphere which is the bottom-most layer of Earth’s atmosphere - and where we all live. There have been two notable historical periods with decades-long episodes of low solar activity. The first period is known as the “Maunder Minimum”, named after the solar astronomer Edward Maunder, and it lasted from around 1645 to 1715. The second one is referred to as the “Dalton Minimum”, named for the English meteorologist John Dalton, and it lasted from about 1790 to 1830 (above).

Both of these historical periods coincided with colder-than-normal global temperatures in an era that is now referred to by many scientists as the “Little Ice Age”. One of the reasons prolonged periods of weak solar activity may be associated with colder global temperatures has to do with a complicated relationship between solar activity, cosmic rays, and clouds on Earth. Research studies in recent years have found that in times of low solar activity - where solar winds are typically weak - more cosmic rays reach the Earth’s atmosphere which, in turn, has been found to lead to an increase in certain types of clouds that can act to cool the Earth.

Outlook This historically weak solar cycle continues the recent downward trend in sunspot cycle strength that began over thirty years ago during solar cycle 22. If this trend continues for the next few cycles, then there would likely be increasing talk of another “grand minimum” for the sun which correlates to an extended decades-long period of low solar activity. Some solar scientists are already predicting that the next solar cycle will be even weaker than this current one which has been historically weak. However, it is just too early for high confidence in those predictions since many solar scientists believe that the best predictor of future solar cycle strength involves activity at the sun’s poles during a solar minimum phase – something we are now rapidly approaching – and the nearly blank look to the sun is likely to become more and more frequent in the months to come.